feature([matTable]): matHeaderRowDef added later is not rendered

[jonahd-g]

Reproduction

Use StackBlitz to reproduce this issue: https://stackblitz.com/edit/angular-uwdzcb

Any headers not specified upon initial rendering are not added. If a matHeaderRowDef is added later on during the lifetime of the component, it will never show up. It seems like the table renderer needs to be altered to keep checking for changes to the present matHeaderRowDef instances.

Steps to reproduce:

1. Click the "Click to Add Rows" text
2. Look at table head rows

Expected Behavior

A second header row should be visible, with a column titled "Name of the Purchased Item"

Actual Behavior

The second header row is not recognized and rendered, even if renderRows() is called.

Environment

• Angular:9.1.3
• CDK/Material: 9.2.1
• Browser(s): Chrome
• Operating System (e.g. Windows, macOS, Ubuntu): gLinux

[crisbeto]

@andrewseguin should have more context around it, but it seems like headers created later on can be included through MatTable.addHeaderRowDef.

[jonahd-g]

Thank you @crisbeto , this may help me create a solution in the interim. Looks like there are methods for adding and removing header row definitions.

https://github.com/angular/components/blob/e512581/src/cdk/table/table.ts#L594

I supposed I'd need to define the headers outside of the matTable component in ng-template elements, then use ViewChildren to acquire them and pass them into the table.

[jelbourn]

I believe this is working as expected currently, so support dynamic column/row definitions via the template API would be an additional feature

[jonahd-g]

That's reasonable. I think the intuitive behavior is that it would be supported. Thus it would be helpful to explain this non-obvious behavior in the documentation.

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